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Volatile components enter the atmosphere, oceans, and other surface waters through pre-, syn-, and post-eruptive volcanic processes that involve a variety of fluids. Aluminosilicate-poor fluids include aqueous or carbonic to sulfide-, sulfate-, chloride-, fluoride-, carbonate-, and phosphate-rich compositions in volcanic environments, but other more complex combinations of these constituents may be involved (Giggenbach 1977; Roedder 1984, 1992; Lowenstern 1995). Other, rare gases and dissolved constituents (e.g., H2, N2, He, Ar, H3BO3, Hg, CH4 and other hydrocarbon compounds, metals, and metalloids) are also present, but in general they are not sufficiently concentrated to form their own phases or control bulk-fluid composition and volcanic processes. As a result of their diverse compositions, volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity, dielectric behavior, viscosity, and other chemical and physical characteristics (Geiger et al. 2006a). The density of a saline liquid in the system NaCl-H2O at 50 MPa and 800 °C, for example, is more than 7 times that of the coexisting aqueous vapor (e.g., 0.2 gm/cm3) (Henley and McNabb 1978).
Multiple fluids move, mix, and/or unmix in magma chambers and volcanic conduits, in the root zone of fumaroles, and in the convective hydrothermal systems that are ubiquitous to volcanic environments. Hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits. Hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant, shallow-crustal pressure-temperature-composition conditions. Volcanic fluids occur at supersolidus to subsolidus conditions, meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present. This chapter addresses the evidence for and occurrence of two or more low-density, aluminosilicate-poor fluids …